Avoiding head immersion with pfd can double the time you have before hypothermia sets in, based on USCG conclusions at end of pdf http://www.uscg.mil/pvs/docs/coldwater1.pdf
. Also a head above water much delays mental fogginess.
Somewhere, a long time ago I read (The Rise and Fall of the Third Reich
?) about how the Nazis repeatedly froze captured Polish pilots to death in cold brine - studying the process of dying of hypothermia. .... One conclusion was that keeping the base of the brain above a certain temperature was absolutely critical - and the fallout was insulated headgear for German pilots. ..... Personally, I do not go out on cold water without a neoprene balaclava - not necessarily on, but pulled down around my neck so it can be raised quickly.
As for swimming speed, I wonder if footwear is a major speed killer. I know bumptoe sandals can almost neutralize the kickstroke needed to reboard a kayak; maybe a compromise is to clip footwear to your pfd cheststrap and do a backstroke. If max swimming speed is needed, I suppose you could trail a pfd on a lanyard.
It's got to depend on the footwear. .... To reduce it to the ludicrous, mountaineering boots clearly inhibit swimming..... but the beach shoes that I used to wear for windsurfing and which I wear when paddling my surf ski actually enhance swimming..... it's like wearing very small swim fins.
Those of us who grew up playing in frigid water may cringe at all the abuse of the term hypothermia. Supposedly anyone in the slightest cooling discomfort is a time bomb facing apocalypse at body temp 95f, but we have spent extended times in bitterly cold water, sometimes with ice in it, and without wimpy wetsuits which I have never used in my life. I think it's an attitude of how you adapt to cold shock, and in an accident you shouldn't prematurely give in to it just because it's the stereotype - fight back.
There are huge, huge differences among people's tolerance for cold water....Something about "brown fat"..... Lynn Cox, for instance, swam the Bering Strait in 43-44F water basically naked. .... And she was in the water for a little over 2 hours. ..... https://en.wikipedia.org/wiki/Lynne_Cox
That being said, I don't count myself among the lucky ones and the "Rule of 50's" rings true to me: 50 yards to swim, 50-degree water, 50-50 chance a naked swimmer is going to survive.
To me, the basic fact of life around cold water is dressing for a certain period of immersion. ..... "Certain Period" because you're going to die eventually - it's just a matter of time.... but after a paddling session, I will typically get off in neck-deep water and fiddle around with my ski for as long as it takes for the cold to really hit home.... that keeps me more-or-less calibrated. ..... I think that some of the most dangerous days around cold water are those beautiful days when the air temp is up in the sixties - but the water is still lethal. ..... On those days, one tends to overheat when paddling hard... and is tempted to dress more for comfort in the air than survival in the water.
it can take 30 minutes or more for most adults to become even mildly hypothermic in ice water.
Then after 30-120 minutes your head fogs and you drown, or if you have pfd you last up to 180 min til cardiac arrest. Actually some of these sources bring up interesting factors based on who you are or what you do that can improve or worsen this timebomb script.
Seems like the tradeoff is between time in the water and the speed with which one loses heat......i.e. I can conceive of a choice between towing my PFD and maybe making it shore in time and just floating around out there and dying in the water......
Also, just to end on a cheery note, there are stories of people who actually made it to shore - and then died some time later because, by the time they got out of the water, they had lost so much heat and their metabolism had slowed so much that their body could not generate enough heat to get them back up to operating temperature.
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